Heretofore, a flexible polyurethane foam (hereinafter referred to as a flexible foam) having a low impact resilence, namely, of low resilence is used as a shock-absorbing body, a sound-absorbing body or a vibration-absorbing body. Further, it is known that when such a foam is used for a cushion material of a chair or for a mattress, body pressure distribution becomes uniform, whereby fatigue feeling and a decubitus can be relieved. For example, a flexible foam of low resilence as disclosed in JP-A-11-286566 is known.
The above document discloses a flexible foam of low resilence, which can be obtained by reacting a urethane foam composition comprising a polyol (a), a polyisocyanate (b), a catalyst (c) and a blowing agent (d), and which has at least one glass transition point within each of a temperature range of from −70° C. to −20° C. and a temperature range of from 0° C. to 60° C., such that when the above glass transition point is represented by a peak value of tan δ obtained by measurement of the dynamic viscoelasticity at a frequency of 10 Hz, the peak value of tan δ within the temperature range of from −70° C. to −20° C. is at least 0.15 and the peak value of tan δ within the temperature range of from 0° C. to 60° C. is at least 0.3.
In this prior art, in order to obtain a flexible foam of low resilence, which is excellent in low resilence at room temperature and shows little increase of hardness at a low temperature, a polyol (a-1) having an average number of functional groups of from 1.5 to 4.5 and a hydroxyl value of from 20 to 70 mgKOH/g and a polyol (a-2) having an average number of functional groups of from 1.5 to 4.5 and a hydroxyl value of from 140 to 300 mgKOH/g are used as the polyol (a), and from 32 to 80 wt % of (a-1) and from 20 to 68 wt % of (a-2) are blended for use. Thus, a flexible foam having at least one glass transition point within each of a temperature range of from −70° C. to −20° C., preferably from −50° C. to −25° C., and a temperature range of from 0° C. to 60° C., preferably from 30° C. to 55° C., is formed.
However, in a method for blending at least two kinds of specific polyols having different hydroxyl values to form a flexible foam as in the above prior art, there was a problem that a hardness change due to a temperature change could not be sufficiently controlled. Further, in recent years, a level of durability required for a flexible foam has been raised. And further, it has been desired to further lower the impact resilience (in Examples of the above document, the impact resilience is somewhat high at a level of from 5 to 18%) to develop a low-resilience where the impact resilience is approximately 0.
The resilience (the impact resilience) of an urethane foam can be usually reduced by blending a plasticizer in a flexible foam, and by adding a proper amount of the plasticizer, a flexible foam having a certain degree of desirable low resilience can be obtained. However, in use, there is a possibility that the added plasticizer is volatilized and absorbed in a human body, and thus, a substance harmful to a human body can not be used. Further, a plasticizer was likely to elute during washing of the foam, whereby it was difficult to maintain the low-resilience of the foam after it was washed repeatedly.
The present invention has been made under these circumstances, and it is an object of the present invention to provide a flexible polyurethane foam, which is excellent in durability, and whereby an excellent low-resilience is obtainable without using a plasticizer, and the hardness change due to a temperature change is little, and a process for producing the same.